The present invention relates to scanning probe microscopy and, more particularly, to a scanning probe microscope having a scan-correction system.
Nearly all scanning probe microscopes, such as scanning tunneling microscopes (STMs), atomic force microscopes (AFMs) and near field acoustic microscopes, use piezoelectric actuators for generating the scans. Piezoelectric materials offer many good properties for such applications: high resolution, high mechanical resonance frequencies, reasonable scan ranges, physical compactness, and low power consumption. A typical piezoelectric tube scanner used in an AFM has a resolution of 1 .ANG., a resonance frequency of 2 kHz, and a scan range of 20 .mu.m. Unfortunately, piezoelectric elements have some undesirable properties resulting in non-linear effects. These distortions take three basic forms: hysteresis (where the steady-state scanner position for a given control input is a function of the past history of movement), creep (where the scanner motion slowly drifts in the direction of recent movements), and non-linear response (where the position of the scanner is not a linear function of the control input).
These non-linear effects make accurate measurements based on scanning probe images very difficult to obtain. For atomic-scale imaging, though, these non-ideal characteristics are not too severe for two reasons: (1) the non-linear behavior of the scanner is small for small scans, and (2) most of the important dimensional parameters, such as bond lengths, are already well known through other measurement techniques. However, for larger-scale imaging applications, such as imaging microfabricated structures, these non-linear effects can create significant distortions in the images. For example, several types of errors generated in images by them include: image "stretching" from one side of the image to the other; nonrepeatable imaging, where the actual scan area depends on the hysteresis of the scanner; and overall image sizes that do not scale linearly with the magnitude of the control input.
One method for reducing these errors is to compensate for known scanner distortions by applying a complex control signal to the scanner. This technique is somewhat helpful, but the number of parameters needed to adequately compensate for the distortions is large. The distortions depend upon the scan speed and waveform, scan size, and the biasing conditions of the scanner's piezoelectric elements. Another method which has been implemented uses the charge on the piezoelectric element, rather than the applied voltage, as the control signal. See, for example, the paper entitled "Application of capacitor insertion method to scanning tunneling microscopes" by Hiroshi Kaizuka which appeared in Vol. 60, No. 10, pages 3119 et seq. of the Review of Scientific Instruments (1989). Unfortunately, this method reduces the available scan range considerably and does not, even theoretically, eliminate the non-linearity completely. The most promising method for correcting the scanner non-linearities is to measure the sample or scanner position with a sensitive detector and then to use this information to correct the non-linearities. A relatively complicated heterodyned interferometric arrangement for measuring the scanner position is described in the paper entitled "Linewidth Measurement by a New Scanning Tunneling Microscope" by Yamada, et al., appearing on pages 2402-2404 in the Japanese Journal of Applied Physics, Vol. 28, No. 11 (1989). The complexity of this arrangement adds to its cost and makes the same unwieldy. Another arrangement requiring direct connection to the sample is disclosed in the paper entitled "Near-field optical scanning microscopy with tunnel-distance regulation" by Durig, et al., on pages 478 et seq. of the IBM Journal of Research and Development, Vol. 30, No. 5 (1986).